First Principles Investigation Of Point Defects And Electronic Structure In Ti₂AlN And M₂AlC（M=V、Cr、Nb And Ta）

Unique combination of metallic and ceramic properties makes the MAX phase materials possible for applications in high-temperature or other extreme environments such as nuclear reactor. In this paper, we use first principles calculations to investigate the defect formation energy and the impurities of H, He and O in Ti₂ AlN and M₂AlC（M=Ti, V, Cr, Nb and Ta） with their electronic structures.The electronic structures of Ti₂ AlN and Ti₂ AlC show that they have good electrical conductivity, which is mainly contributed by the Ti element. There are Ti-C（Ti₂AlC）,TiN（Ti₂AlN） convalent and Ti-Al metallic bond in Ti₂ AlC and Ti₂ AlN. The vacancy formation energies for Ti₂ AlN are in the sequence > > , their migration energies are ₂.835 eV（Ti）, 0.68eV（Al） and 3.3₂ eV（N） respectively. It is same as the Ti₂ AlC that the Al mono-vacancy is the most energetically favorable in Ti₂ AlN. They are usually observed diffusing towards the surface to form a protective Al₂O3 layer. The H, He and O impurities show that O impurity is most energetically favorable to form in Ti₂ AlC and Ti₂ AlN. These results are fairly accorded with experimental results reported in literatures.We also calculated the mono-vacancy formation energy of V₂ AlC, Cr₂ AlC, Nb₂ AlC and Ta₂ AlC. The Cr vacancy formation energy is relatively small compared with the Al and C vacancy in Cr₂ AlC. The M vacancy formation energies are maximum, however, the C vacancy formation energy were minium in V₂ AlC, Nb₂ AlC and Ta₂ AlC. These vacancies have a slight effect on the bonding and electronic properties of M₂ AlC, which suggest that these materials still have a good mechnical and electrical properties.These conclusions could help us from a micro perspective of MAX phase materials. We also understand the defects fomation mechanism in M₂ AX materials by calculation the defect formation energies. Ti₂ AlC and Ti₂ AlN have a better performance than V₂ AlC, Cr₂ AlC, Nb₂ AlC and Ta₂ AlC, and are more suitable for application under high temperature or irradiation envirouments.